It is well known in the polymer field that .alpha.-olefins can be polymerized to form normally solid polymers possessing widely differing physical properties, such as melt flow, tensile strength, rigidity and impact strength. Depending upon the level of the particular properties, these polymers are utilized in the formation of fibers, films and molded articles. There are occasions, however, when it is desirable to improve one or more of the physical properties of these polymers for a particular purpose. Many materials have been added to the polymers in efforts to achieve such improvements in the properties. While some specific physical property may be improved in the resulting composition, it is usually at the sacrifice of some other property or properties.
For example, polymers of propylene have been used extensively in the fabrication of molded parts. It is also generally recognized that these polymers are relatively flexible and lack the rigidity required for some end uses, particularly where the fabricated parts are subjected to a relatively high temperature. Specifically, although polypropylene would be an inexpensive core material for steel-plastic laminates in the automotive industry, it will not withstand the high temperatures (375.degree. F.) of the paint baking cycle customarily used in fabricating automotive products. Furthermore, these polymers of propylene lack impact resistance as shown by their low notched Izod impact values. Because of these deficiencies in impact resistance, it has not been possible to substitute lower cost polypropylene for higher cost polymers such as acrylonitrile-butadiene-styrene (ABS).
In order to improve the impact resistance of a polymer such as a propylene homopolymer or propylene copolymers with ethylene and/or other monomers, the polymer is blended with an elastomer. The addition of the elastomer does increase the impact strength of the polymer. However, such addition of elastomer to polymers of propylene result in a lower heat deflection temperature and a loss of rigidity, that is to say the modulus of elasticity is adversely affected. It is also known that additions of different elastomers to the same polymer have varying effects on the strength of the polymer.
Frequently inexpensive fillers are used in polyolefin resins as an extender to replace volume for volume the relatively more expensive polyolefin. When filler is added to a polyolefin, certain properties such as rigidity and heat deflection temperature may be improved. Again such additives often reduce the impact resistance and adversely affect the level of the other properties of the polyolefin composition such that the composition will no longer make satisfactory products.
The prior art discloses compositions comprising additions of both elastomers and fillers to polyolefins in attempts to utilize, in combination, the ability of the individual additives to augment various properties of the particular polyolefin. U.S. Pat. No. 3,572,721 discloses, in general, a blend of rubber and a thermoplastic for use in producing molded play balls, particularly golf balls having good resistance to cutting and producing an acceptable "click" when struck with a golf club. Very broadly disclosed, the blend comprises a substantially crystalline homopolymer or copolymer of an .alpha.-olefin and a rubber in an amount of 5 to 95 percent by weight of the blend but, practicably, a molded play ball is 50 to 90 percent by weight rubber. Depending on the size and weight of the ball to be produced filler material from 30 to 70 percent by weight of the polymer can be added.
U.S. Pat. No. 3,860,542 describes a particular propylene resin composition that yields moldings claimed to have excellent mechanical properties such as impact strength, flexibility and flame-retardance. In this embodiment of the invention, the propylene resin composition comprises 40 to 5 parts by weight of a propylene polymer and 60 to 95 parts by weight of alumina trihydrate having a gibbsite crystal structure as the filler and 5 to 15 parts by weight, based on 100 parts by weight of the propylene polymer and filler, of one or more high molecular weight organic substances having a softening point below room temperature.
Polypropylene compositions having improved stiffness are revealed in U.S. Pat. No. 3,872,052. This patent discloses that additions of an impact modifier improves the impact resistance and decreases the stiffness of polypropylene compositions and that addition of acicular hydrated sodium aluminum carbonate is superior to other fillers in imparting stiffness.
U.S. Pat. No. 3,963,647 describes an injection moldable, electrostatically paintable polyolefin composition comprising 20-100 parts of a crystalline propylene polymer, 100 parts of an ethylene-higher .alpha.-olefin copolymer elastomer and a carbon black filler. U.S. Pat. No. 4,002,595 discloses directly electroplatable polypropylene compositions comprising a propylene homopolymer or copolymer, an ethylene homopolymer or copolymer, a low polarity rubber, carbon black filler and, optionally, a silicate mineral additive.
While the art discloses compositions containing a polyolefin, an elastomer and a filler having various improved properties, there nevertheless remains a need for polyolefin resin blends that have exceptional impact resistance over a limited compositional range while incorporating relatively high levels of inexpensive filler.
There is also a need for polyolefin resin blends that have exceptional impact resistance and good high temperature flow resistance.
There is a further need for polyolefin resin compositions that offer a cost advantage over the high impact polystyrene resins while affording comparable or superior properties.
There is a still further need for a polyolefin resin composition that can be used as an inexpensive core material for metal-plastic laminates.
Specifically, there is a need for polypropylene compositions having high impact strength and good resistance to flow at automotive paint baking temperatures for use in steel-plastic laminates in the automotive industry.